The present invention generally relates to electrical power conversion and, more particularly, to synchronizing the chopping frequency of multiple power converters and providing a phase shift to control voltage ripple across the DC bus.
The electronics for supplying power in aerospace applications plays a significant role in the modern aircraft and spacecraft industry. This is particularly true in the area of more electric aircraft or more electric architecture (MEA) for aircraft and military ground vehicles. MEA includes, for example, a concept called “power-by-wire,” in which electrical power moves (i.e., “actuates”) aircraft flight control surfaces such as rudder and aileron. MEA can eliminate the complex, heavy, maintenance-intensive, and (in combat) vulnerable hydraulic systems with their flammable liquids operating at high temperature and pressure. Using MEA, the weight of miles of tubing, the pumps, and valves can be shifted from plumbing to passengers, fuel or mission payloads.
The commercial aircraft business is moving toward non-bleed air environmental control systems (ECS), variable-frequency (VF) power distribution systems, and electrical actuation. Typical examples are the latest designs, such as the Boeing 787 and the Airbus super jumbo A380. The next-generation Boeing airplane (replacement of 737) and the Airbus airplane (replacement for the A320) will most likely use MEA. Some military aircraft already use MEA, including primary and secondary flight control. Military ground vehicles have migrated toward hybrid electric technology where the main propulsion employs electric drives.
These developments have resulted in a substantial demand for electrical power conversion. For example, non-bleed air environmental control systems need additional electric drives for vapor cycle system (VCS) compressors, condenser fans, and liquid pumps. Also, a large number of electric drives for fans is required. In constant-frequency applications, these fans have predominantly used direct drive (i.e., no power electronics) to an induction machine. The new architecture presents a need for double power electronics conversion ac-to-dc and dc-to-ac. In addition, auxiliary power unit (APU) and main engine electric start impose a need for high-power, multiple-use electric power controllers. Moreover, military aircraft require high-voltage (270-Vdc) power conversions multiple times, for example, from generator power to power for electric flight controllers and utilization. Furthermore, military ground vehicles have moved toward a higher voltage power distribution system where high-power bidirectional propulsion is being used for driving and dynamic braking. The power generation is typically achieved by a main engine shaft driving one or more large electric machines, requiring bidirectional conversion for power conditioning and self-starting.
In summary, there is a need for power converters and motor controllers for aircraft and ground military and commercial applications for: 1) increased power level conversion capabilities to handle increased loads; 2) reduced controller weights to be able to accommodate large power electronics content increase per platform; 3) reduced volume to accommodate electronics housings in limited compartment space; 4) increased reliability for achieving reasonable mission success; and 5) reduced cost for affordability.
A switching type power electronics converter requires a low-impedance source in close proximity to high power switched modules (HPSM) in order to provide proper operation. A capacitance bank connected in parallel with the direct current (DC) bus performs the function of providing the low-impedance. In some cases more than one capacitor is used to achieve a better distribution of the low impedances across the switching devices. The selection of the value of these capacitors primarily depends on the magnitude of the switched currents and the switching frequency. The capacitor bank experiences charging and discharging cycles, which are synchronous with the switching period. These charging and discharging cycles create voltage ripple across the bus capacitor, i.e., capacitance bank, due to the source impedance, distribution bus impedance and the electromagnetic interference (EMI) filters. The ripple amplitude also depends on the duty cycle of the converter, which is directly related to the loading. The voltage ripple across the capacitance bank creates two negative effects: 1) capacitor alternating current (AC) that creates undesired heating and 2) EMI radiated and conducted emission effect. These effects can be controlled by reducing the voltage ripple and are required to be controlled for the system to meet specifications. Thus, reduction of the voltage ripple is a useful result.
As can be seen, there is a need for a method for DC bus voltage ripple compensation to reduce voltage ripple for power conversion units. A voltage ripple compensation method is needed that improves performance, reduces cost, reduces weight and volume, and improves reliability.